1. Field of the Invention
The present invention relates to an image pickup apparatus and, more specifically, to the technology of extending a dynamic range.
2. Description of the Related Art
There are technologies of extending a dynamic range in image pickup apparatuses. Japanese Patent Laid-Open No. 2006-197382 (hereinafter referred to as Patent Document 1) discloses an image pickup apparatus that aims at extending a dynamic range without a decrease in image quality. Specifically, the image pickup apparatus includes a transfer-gate control portion that controls a potential of a transfer gate to introduce a part of charges flowing from a photoelectric conversion portion into a floating diffusion portion and an image-signal generating portion that generates an image signal based on charges stored in the photoelectric conversion portion and charges flowed into the floating diffusion portion.
Japanese Patent Laid-Open No. 2008-099158 (hereinafter referred to as Patent Document 2) discloses a configuration in which intermediate voltages having the same voltage value are supplied a plurality of times as a control voltage from a driver circuit to a gate electrode of a transfer transistor in synchronization with column selection and at that time signal charges transferred by the transfer transistor are read out at least twice. Each of the intermediate voltages supplied to the transfer transistor is transferred to a floating diffusion (FD) region, and a potential in the FD region is read out as a signal level. In a different embodiment, a configuration using a mechanical shutter is disclosed. In that configuration, a dummy transfer is performed during the opening of the mechanical shutter, and an intermediate voltage is transferred and read out during the closing of the mechanical shutter.
One possible method for extending a dynamic range in an image pickup apparatus is an increase in saturation charge quantity of the photoelectric conversion portion. However, if the saturation charge quantity of the photoelectric conversion portion is simply increased, not all charges may be read out in a readout circuit disposed downstream of the photoelectric conversion portion or a signal based on a readout charge may be unable to be used in image formation.
In Patent Document 1, light is incident on the photoelectric conversion portion, charges flowed from the photoelectric conversion portion during the period of generating signal charges for use in image formation (the exposure period) in the photoelectric conversion portion are transferred to the FD region. That is, on the assumption of generation of charges exceeding the saturation charge quantity of the photoelectric conversion portion during the exposure period, charges are transferred during the exposure period. Thus, if strong light is given after a last readout operation during the exposure period and charges are stored near the saturation charge quantity of the photoelectric conversion portion, not all charges may be used in image formation depending on the dynamic range of the downstream readout circuit.
In Patent Document 2, if strong light is entered, a dummy transfer is performed and a part of generated charges is ejected through a reset switch. Therefore, there may be an issue in terms of signal continuity. In addition, no discussion is made about variations in transfer efficiency from the photoelectric conversion portion depending on temperature conditions of the exterior and the gain of the readout circuit disposed downstream of the photoelectric conversion portion.